Nacelles are well known in the aviation industry as streamlined enclosures for aircraft engines. Each enclosure ordinarily includes one or more acoustic liners with perforated face sheets for absorbing engine noise, directing airflow around the engine, and providing structural support for the engine. These face sheets can be comprised of composite laminates, which are beneficial because they have substantially lightweight characteristics, desirable resistance to fatigue in sonic environments, substantial formability, and favorable life cycle costs.
The perforated composite laminate sheets can be manufactured by utilizing one or more pinmats, a high-pressure perforation machine, and an autoclave machine, which together perfonn a three-cycle operation. This operation typically includes a lay-up cycle, a perforation cycle, and a cure cycle.
Prior to the lay-up cycle, a contoured tool base is prepared by nesting two or more pinmats together on a surface of the tool base. These nested pinmats can form a desired pin field that defines an area of the face sheet to be perforated. Each pinmat ordinarily includes a thick mat portion and a dense population of pins extending from the mat portion. These pins typically are intended to perforate an uncured laminate sheet. For this reason, the uncured laminate sheet ordinarily is laid on top of the bed of pins that extend from the pinmats.
In the lay-up cycle, a vacuum device normally is utilized for forming an uncured laminate sheet from a series of individual sheets of raw composite materials. Specifically, a predetermined number of the individual sheets are stacked in succession on the surface of the tool base. Thereafter, a vacuum bagging film can be placed over the sheets and sealed against the surface of the tool base. This construction permits air to be drawn from within the bagging film thereby subjecting the stack of sheets to a predetermined amount of compaction pressure, e.g. about 10-14 psi, for a predetermined amount of time. This compaction pressure typically compresses the stack of individual sheets into a single uncured laminate sheet. Once this sheet has been formed, a perforation procedure typically is commenced.
During the high-pressure perforation cycle, the uncured laminate sheet usually is perforated by a high-pressure perforation machine. The uncured laminate sheet usually has a somewhat flexible pressure pad (“perforation pressure pad”) laid thereon. This perforation pressure pad usually is a sheet comprised of a silicone rubber material and is intended to transfer pressure, e.g. about 500-600 psi, from the high-pressure perforation machine to the uncured laminate sheet In this respect, the application of pressure can force the laminate sheet onto the pinmat thereby causing the pins to penetrate the laminate sheet in order to form the desired perforations.
The perforation of the laminate sheet typically requires that the pins displace continuous inelastic fibers within the laminate sheet beyond the paths of the pins. In particular, when the laminate sheet is laid upon the pinmat, the tip of each pin can contact an interface portion of a fiber. As the perforation machine forces the laminate sheet onto the pinmat, each pin typically displaces the interface portion of each fiber laterally outward with respect to the longitudinal axis of that fiber. As a result, the peripheral portions of each inelastic fiber can be drawn toward the interface portion of the fiber generally along the longitudinal axis of that fiber.
The lengthy nature of the fibers can create two conditions which require a substantially high amount of pressure, e.g. about 500-600 psi, for displacing the fibers and forming the perforations. First, each relatively long continuous fiber is firmly held in place by the interlocking nature of the fabric weave and to a lesser degree by a substantial amount of resin that is cast around the diameter of the fiber along the length of that fiber. The degree to which the interlocking fabric weave and the resin firmly bold the fiber in place correlates to the minimum amount of applied pressure required for causing the pin to overcome the grip of the interlocking weave and the resin so as to displace that fiber beyond the pin path. Second, the lattice nature of the woven fabric creates a field tension when the fabric is engaged by a substantial number of pins. This field tension increases the tensile stress within the fiber thereby increasing that fiber's resistance to being displaced by the pins. This resistance to displacement typically opposes pin penetration into the laminate sheet thereby opposing the formation of the perforations.
Moreover, recent studies have shown that the perforation of laminate sheets can also result from the fracture of the laminate fibers within the pin paths. The fracture of laminate fibers is evidenced by the deposits of fiber debris on the pin tips after the perforation cycle. The amount of debris indicates that a substantial amount of the fibers in the pin paths may be broken by the pins.
Specifically, a substantially high amount of pressure can be required to displace a continuous inelastic fiber such that it exceeds a critical stress level within that fiber. This critical stress can cause the fiber to fracture so as to create at least two shorter lengths of fiber from the original fiber. The fracture generally occurs at the location of pin penetration where the induced stresses within the fiber are the highest. After the fiber breaks, tension in that fiber is eliminated, between the nearest two points where that fiber was locked into the fabric weave, e.g. the nearest overlapping tows. As a result, the shorter lengths of fiber can be more easily displaced by the pins thereby permitting the pins to penetrate deeper into the sheet for contacting the next inelastic continuous fiber.
After the perforations have been formed, preparations are made for the cure cycle. In particular, the perforation-pressure pad typically is removed and a second more flexible consolidation pressure pad is placed on top of the pins. Similar to the perforation pressure pad, the consolidation pressure pad ordinarily is a sheet of a silicone rubber material.
In the cure cycle, the tool base and a vacuum bagging film enclose the assemblage of the pinmats, the laminate sheet, and the consolidation pressure pad. Then, this assemblage is placed into an autoclave machine for applying predetermined amounts of heat and pressure, e.g. about 90 psi, to the laminate sheet. This combination of the applied pressure and heat can cause the consolidation pressure pad to slightly deform around the protruding pins and contact the laminate sheet. This engagement between the consolidation pressure pad and the laminate sheet may allow the consolidation pressure pad to transfer a portion of the applied pressure to the laminate sheet so as to compress the laminate sheet between the consolidation pressure pad and the mat portion of the pinmat. This compression causes the laminate sheet to be consolidated. At the conclusion of the cure cycle, the perforated composite laminate sheet is produced.
A drawback of the existing perforation system is that the pins typically have conical tips that require the high-pressure perforation machine to apply substantial amounts of pressure in order to cause the pins to perforate the laminate sheet. These conical tips typically have one or more smoothly curved surfaces that substantially distribute stresses within each fiber along its longitudinal axis near the pin-laminate engagement. In other words, the construction of each pin tip typically fails to concentrate stress at the interface portion of the fiber. As a result, the fiber can be subjected to nearly the full amount of its critical tensile stress before failing. For this reason, a relatively high amount of pressure typically must be applied to the laminate sheet in order to fracture and displace the fibers. This relatively high amount of pressure usually cannot be provided by an autoclave machine. Instead, a separate machine, e.g. the high-pressure perforation machine, normally is utilized for applying the substantially high amounts of pressure. This additional machine usuajly requires a substantial amount of time to complete the perforation cycle thereby increasing manufacturing cycle time, as well costs associated therewith. Such results clearly are undesirable.
Another drawback of the existing system is that the silicone rubber pressure pads usually lack sufficient elasticity for adequately deforming around the pins and consolidating the laminate sheet against the pinmat during the cure cycle. Instead, the pressure pad typically transfers a substantial portion of the pressure to the pins thereby inadequately consolidating the perforated regions of the laminate sheet. This poor consolidation can cause those perforated regions to have undesired porosity and hence lower strength characteristics.
Additionally, the inadequate consolidation of the perforated regions can create a pressure differential between the perforated regions of the sheet and the adjacent non-perforated regions of the sheet. In particular, a greater portion of the autoclave pressure can be transferred to the non-perforated regions of the laminate sheet while a smaller portion of the autoclave pressure is transferred to the perforated regions. This pressure differential typically causes resin within the laminate sheet to locally migrate from the higher-pressure non-perforated regions to the lower-pressure perforated regions. This resin migration results in the formation of a depression within the sheet along the perimeter of each perforated region. These depressions are disadvantageous because they typically are areas of weakness within the laminate sheet and can require additional finishing operations for repairing the surface of the sheet.
Therefore, a need exists for a method and system for manufacturing a perforated composite laminate sheet that decreases pressure requirements for the perforation of the laminate sheets, improves the consolidation of the laminate sheets, and decreases the manufacturing cycle time, as well as the costs associated therewith.